Starter and discharge lamp starting circuit

ABSTRACT

A glow bottle starter having three lead-in wires and containing two bimetals connected together at one end and a rigid tungsten rod. The bimetals make normally closed contact with the tungsten rod in the quiescent state of the starter, and upon electrical current flow through one or both of the bimetals to cause elevation to a predetermined temperature, the bimetals are operative to separate from the tungsten rod. In a circuit including the glow-bottle starter for starting a discharge lamp, one of the bimetals and the rigid tungsten rod are connected across the output of a ballast circuit, and the tungsten rod and other bimetal are connected across the lamp. Upon initial energization of the ballast, short circuit current through the bimetal connected thereto is operative to separate the bimetals from the tungsten rod to provide an open starter circuit and a switching transient across the lamp; upon starting of the lamp, the resulting current flow through both bimetals maintains the bimetals separated from the rod.

BACKGROUND OF THE INVENTION

This invention relates to glow-type starter devices used for startingdischarge lamps and circuits employing such starters, and moreparticularly, to a starter and starting circuit for use in the operationof high intensity discharge lamps containing noble gases at highpressures, e.g., in excess of 100 torr.

High intensity discharge lamps, such as high pressure sodium lamps,commonly include noble gases at pressures below 100 torr. For example,the starting gas in the standard high pressure sodium lamp is xenon at14 torr. Lamps containing noble gases at pressures below 100 torr can bestarted and operated by utilizing an igniter in conjunction with a lampballast. The igniter electronically provides high voltage, shortduration pulses which assist in initiating discharge. The lamp ballastconverts the AC line voltage to the proper amplitude and impedance levelfor lamp operation.

It has been found that the inclusion in high pressure sodium lamps ofxenon as the noble gas at pressures well in excess of 100 torr isbeneficial to lamp performance. However, the igniter described abovedoes not produce reliable starting at xenon pressures above about 100torr. A conductor wrapped around the discharge tube and connected to oneof the electrodes is described as having been utilized in assisting thestarting of a lamp containing xenon at pressures up to 300 torr in U.S.Pat. No. 4,179,640. The lamp is described as having been operated from aconventional ballast and starting pulse generator.

Another arrangement for starting high pressure discharge lamps is shownin U.S. Pat. No. 4,137,483. A switching circuit contained within thelamp induces a high voltage starting pulse. The high voltage pulseoperates in conjunction with a conductor wrapped around the dischargetube to initiate discharge in the lamp. The igniter and the conventionalballast are not used.

Recent developments have indicated the desirability of including xenonat pressures in excess of 300 torr in high pressure sodium lamps.However, none of the starting arrangements described above are effectiveto reliably start lamps having xenon pressures in excess of 300 torr.For example increases in efficiency of approximately 20% can be obtainedby increasing the xenon pressure to 400-500 torr, but the voltagenecessary to start these lamps is approximately double that required forlamps having a fill pressure below 100 torr. Such high amplitudestarting pulses, however, undesirably increase the dielectric stresseson auxiliary equipment.

An improved starting arrangement for overcoming the aforementionedproblems with respect to the starting of high intensity discharge lampswith fill gas pressures in excess of 300 torr is described in acopending application Ser. No. 139,310, filed Apr. 11, 1980 and assignedto GTE Laboratories Incorporated. A high pressure sodium lamp includinga discharge tube containing xenon at pressures in excess of 300 torr isreliably started by the combination of an igniter, a conductor wrappedaround the discharge tube, and a switching circuit. A conventional lampballast provides ac power during starting and normal operation. Theconductor about the discharge tube is a starting aid which intensifiesthe electric field within the xenon filled tube. The igniter providesperiodic pulses of 2,500-4,000 volts with a duration of about onemicrosecond. The switching circuit provides a high voltage pulse havingan amplitude about equal to the amplitude of the periodic pulses and aduration much greater than the duration of the periodic pulses; e.g., inthe order of 100 microseconds. Hence, reliable starting is achieved byincreasing the pulse duration rather than the amplitude in order tominimize the dielectric stresses on auxilliary equipment.

In the aforementioned copending application Ser. No. 139,310, thespecific means disclosed for implementing the switching circuitcomprises, in one instance, a manual switch in series with a currentlimiting resistor connected across the lamp. A second switchimplementation disclosed in the copending application is illustratedschematically in FIG. 1 and comprises a thermal switch 54 including aheater resistor 58 and a bimetal switch 60 connected in series. Inputs Aand A' from the lamp ballast and igniter are coupled through the heaterresistor 58 to the electrodes of the discharge lamp 50. The bimetalswitch 60 and a current limiting resistor 56 are coupled in seriesacross the electrodes of the discharge lamp 50. In operation, the lampballast provides AC power to the points A and A' and the igniterprovides periodic pulses of high amplitude and short duration to thepoints A and A' as described hereinabove. Also, the conductor 52promotes the formation of an ionization path within the discharge lamp50 as also described above. In a cold condition, the bimetal switch 60is closed. Therefore, when power is applied to the points A and A',current flows through the resistor 58, the bimetal switch 60, and theresistor 56. The heater resistor 58 is placed in close proximity to thebimetal switch so that heat generated by current passing therethroughwill heat the bimetal switch 60. After a predetermined time, the heatgenerated by the resistor 58 causes the bimetal switch 60 to switch tothe open position and the current drawn from the ballast is rapidlydecreased. The rapid decrease in current drawn from the ballast causesthe highly inductive output of the ballast to generate a high voltagepulse which provides sufficient energy to initiate discharge in the lamp50. The current drawn by the discharge lamp 50 through the resistor 58causes the resistor 58 to remain heated and the bimetal switch 60 toremain in the open position. If for some reason, the discharge lamp 50does not start when the bimetal switch 60 opens, no current is drawnthrough the resistor 58, and the bimetal switch 60 cools until itrecloses. Heating of the resistor 58 again occurs, causing the bimetalswitch 60 to open and another high voltage starting pulse is generated.Thus, the starting process is repeated until a discharge is initiated inthe lamp 50.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a new and improvedstarter device and an improved circuit for starting a high intensitydischarge lamp.

A further object of the invention is to provide improved means forstarting high intensity discharge lamps containing a noble gas ormixtures thereof at pressures in excess of 100 torr.

These and other objects, advantages and features are attained, inaccordance with one aspect of the invention, by providing a glow-starterdevice comprising an hermetically sealed glass envelope having threelead-in wires extending therethrough and containing a gas atsubatmospheric pressure. A first bimetal within the envelope is mountedon a first one of the lead-in wires, and a second bimetal is mounted ona second one of the lead-in wires, with the bimetals being electricallyand mechanically connected together at one end. A rigid conductivemember within the envelope is mounted on the third lead-in wire, and thebimetals are disposed to make electrical contact with the rigidconductive member at the connected-together end of the bimetals atnormal room temperature and in the absence of energization of thelead-in wires. In operation, the bimetals are operative to separate fromthe rigid conductive member at a predetermined elevated temperature suchas caused by electrical current flowing through one or both of thebimetals.

According to another aspect of the invention, a discharge lamp startingcircuit is provided which comprises; a ballast circuit having an inputconnected to a source of AC line voltage and a power output; aglow-starter device having a first terminal connected to a firstbimetal, a second terminal connected to a second bimetal, and a thirdterminal connected to a rigid conductive member, the bimetals beingelectrically connected together at one end which makes a normally closedcontact with the rigid member in the quiescent state of the device;means connecting the second and third terminals of the starter deviceacross the output of the ballast circuit; and means for connecting thefirst and third terminals of the starter device across the terminals ofa discharge lamp. Upon initial energization of the ballast, shortcircuit current through the second and third terminals of the starterdevice is operative to flex the second bimetal for separating thebimetals from the rigid member to provide an open circuit thereat and aswitching transient across the lamp. Upon starting of the lamp, the lampcurrent flow through the first and second terminals of the starterdevice is operative to maintain the bimetals separated from the rigidmember.

The circuit is particularly useful where the lamp is a high pressuredischarge lamp including an hermetically sealed arc tube containing anoble gas or mixtures thereof having a pressure greater than 100 torr.The preferred ballast circuit provides an ac output and includes anigniter for providing periodic high voltage pulses substantiallysynchronized with the peaks of the AC output of the ballast circuit. Insuch an arrangement, each separation of the bimetals from the rigidmember in the starter device is operative to produce a high voltagepulse across the lamp which has a substantially longer duration than theperiodic pulses of the igniter.

Accordingly, the starting circuit of the present invention represents animprovement over the circuit described in the aforementioned applicationSer. No. 139,310 in the following respects. The present circuiteliminates the need for the current limiting resistor 56 and the heaterresistor 58 of FIG. 1. Instead of the heater resistor, the bimetals ofthe present switching device are self heated by means of the bimetal I²R characteristic. The bimetal switch is then maintained in the opencondition during lamp operation by virtue of the defined circuitarrangement with a unique three terminal glow starter device. Furtheryet, the starter device of the present invention permits greater controlover the amplitude of the starter actuated transient pulse as this pulseamplitude can be quite closely controlled by selection of theglow-bottle gas and pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be more fully described hereinafter in conjunctionwith the accompanying drawings, in which:

FIG. 1 is a circuit diagram of a prior art discharge lamp startingarrangement to which reference has been previously made;

FIG. 2 is an elevational view of a glow starter device according to theinvention;

FIG. 3 is a combined block and circuit diagram of a discharge lampstarting circuit employing the starter device of FIG. 2, in accordancewith the invention; and

FIG. 4 is a graphic illustration of the voltage waveform applied to theelectrodes of the discharge lamp shown in FIG. 3.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to FIG. 2, a glow starter device 10 according to theinvention comprises an hermetically sealed glass envelope 4 havinglead-in wires 1, 2 and 3 extending therethrough. More specifically, allthree lead-in wires are sealed through a reentrant stem 5 sealed at thebase of the glass envelope 4.

Within the envelope, a first bimetal 6 is mounted on the lead-in wire 1,and a second bimetal 7 is mounted on the lead-in wire 2, in each casethe bimetal being welded at one end to the inner end of the respectivelead-in wire so as to provide a mechanical and electrical connectionthereto. In the embodiment illustrated, the bimetals are shown asstrips, and at the opposite end of the strips from that connected to thelead-in wires, the bimetals are electrically and mechanically connectedtogether, such as by a segment 8 of conductive material. Attached, suchas by welding, to the inner end of the lead-in wire 3 and disposedwithin the envelope is a rigid conductive member 9.

The flexible bimetal strips 6 and 7 are disposed within the envelopesuch that in the quiescent state of the device, i.e., at normal roomtemperature and in the absence of energization of the lead-in wires, theconductive segment 8 at the connected-together end of the bimetals 6 and7 functions as a contact member which is resiliently urged against therigid conductive member 9 by the bimetals to make electrical contacttherewith. As will be further discussed hereinafter, the bimetals 6 and7 are then operative to deflect away and separate from the rigidconductive member 9 at a predetermined elevated temperature, forexample, 160° C., such as may be caused by the I² R characteristics ofthe bimetals upon electrical current flowing through one or both of thebimetals 6 and 7. Thus, a sufficient level of electrical current flowingthrough one or both of the bimetals 6 and 7 can cause a break in theelectrical contact between segment 8 and conductive member 9.

According to one specific implementation of a glow-bottle starter deviceaccording to the invention and intended for use in starting high wattagedischarge lamps, i.e., above 100 watts, the envelope 4 was formed ofsoda lime glass having an outside diameter of about 15 millimeters and alength of about 13/4 inches. Each of the bimetal strips 6 and 7comprised Chase 3900 material having a thickness of 0.005 inch, a widthof 0.140 inch and a length of 13/16 inch. Lead glass was used for thereentrant stem 5, and the lead-in wires 1-3 were of nickel. Conductivemember 9 was a tungsten rod having a diameter of 0.045 inch and a lengthof 13/16 inch, and the conductive contact 8 also comprised a segment oftungsten rod. The tension of the bimetals in forcing contact segment 8against the rod 9 was between about 40-50 grams. The hermetically sealedenvelope 4 was filled with argon gas at a pressure of 2 torr. Thisglow-bottle embodiment intended for high wattage lamp starting alsoincludes an insulating glass sleeve 11 disposed about the lower portionof the tungsten rod 9, as illustrated, to prevent unwanted arc over tolower portions of the bimetal strips. This high wattage lamp starterdevice was capable of handling a current flow of up to about 4 amperes.

According to another specific implementation of a glow-bottle starterdevice according to the invention and intended for use in starting lowwattage discharge lamps, i.e., less than 100 watts, the envelope 4 wasformed of soda lime glass having an outside diameter of 15 millimetersand a length of 40 millimeters. The stem 5 was formed of lead glass, andthe lead-in wires 1-3 were formed of nickel. Bimetals 6 and 7 comprised6650 material having a thickness of 0.004 inch, a width of 0.040 inchand a length of 5/8 inch. The conductive member 9 comprised a tungstenrod having a diameter of 0.045 inch and a length of 5/8 inch. In lieu ofthe contact segment 8, this implementation employs a pair ofsilver-plated copper contact buttons, one being attached to the weldedtogether ends of the bimetal strips 6 and 7, and the other contactbutton being attached to the tungsten rod 9. The tension of the bimetalsin forcing the contact button of the strips against the contact buttonsof the rod was about 4 grams. The envelope is filled with argon gas at apressure of about 4 torr. Further, in this embodiment intended for lowwattage lamp starting, there was no necessity for a glass insulatingsleeve about the tungsten rod 9, and the device was capable of handlinga current flow in the order of one ampere.

A particularly useful lamp operating circuit employing the abovedescribed three-lead glow starter device is shown in FIG. 3. The circuitincludes a high intensity discharge lamp 30, the starter device 10, anda ballast circuit 32 which can comprise an igniter 16 and a conventionallead or lag type lamp ballast 18. Although not necessary to theinvention, the particular embodiment illustrated is more suitable for ahigh pressure sodium lamp, with a fill of xenon at high pressure, of thehigh wattage type, i.e., above 100 watts. The lamp includes an arc tube20, which in the case of a high pressure sodium lamp is made of aluminaor other transparent ceramic material, having electrodes 22 sealedtherein at opposite ends and respectively connected to lamp terminals 23at each end. The lamp typically also includes an outer glass jacketwhich is not shown. A conductor 12, typically a fine wire, is wrappedabout the arc tube 20 and is coupled to the electrode 22 which isconnected to a reference potential such as ground. As will be made clearhereinafter, the conductor 12 functions as a starting aid for the lamp.

The lamp ballast 18 has an input connected to a source of AC linevoltage, typically 115 volts, 60 Hertz, and has a power outputrepresented by lines 24, connected across the lamp terminals 23. Theigniter 16 receives an ac input from an auxillairy output of the lampballast 18. Outputs 26 of the igniter 16 are also coupled to the lampterminals 23.

The above described glow starter device 10 is connected in the circuitas follows. Terminals 1 and 3, associated with bimetal 6 and the rigidconductive member 9, respectively, are connected across the terminals 23of the discharge lamp. Terminal 2, associated with bimetal 7, andterminal 3 are connected across the output of the ballast circuit 32,i.e., across the junctions of lamp ballasts output lines 24 and igniteroutput lines 26.

The arc tube 20 encloses a fill material, typically including sodium ora sodium amalgam and a noble gas or mixtures of noble gases, which emitlight during discharge. In particular, the inclusion in the arc tube 20of xenon at pressures above 300 torr provides superior lamp performance.Lamp ballasts for high intensity discharge lamps are well known in theart and can be of the leading or lagging type. One example of a suitablelamp ballast is General Electric Model No. 17G3202. The lamp ballast 18is operative to step the AC input voltage up or down depending on themagnitude of the AC input and to provide a relatively high impedanceoutput. The igniter 16, also well known in the art, is operative toprovide high amplitude, short duration pulses which assist in initiatingdischarge in the discharge lamp 30. Pulses appearing at outputs 26 aretypically 2500 to 4000 volts in amplitude and at least one microsecondin duration. Furthermore, the pulses occur in timed relation to the ACpower, typically being substantially synchronized with the peaks of theAC voltage. The specifications for the pulses produced by the igniterhave been standardized by the American National Standards Institute inspecification ANSI C78.1350-1976. One example of a suitable igniter isGeneral Electric Model No. 17G9932.

The combination of the conductor 12, the starter device 10, and theigniter 16 form a starting circuit which is operative to initiatedischarge in the discharge lamp 30 while the lamp ballast 18 provides ACpower on a continuous basis during starting and normal operation. Theinitiation of a discharge in the lamp 30 can be described as followswith reference to FIG. 4. Assume in the present example that the ACpower is applied prior to the opening of the starter 10 contacts(segment 8 and rod 9). Thus, the lamp voltage remains approximately zerountil time T_(o) when the starter contacts separate to provide an opencircuit. The opening of the starter contacts causes an inductivelygenerated high voltage pulse 38 to be applied to the lamp. After theopening of the starter contacts, the lamp ballast 18 provides at theoutputs 24 an AC voltage 40, typically 180 volts AC for a 400 watt highpressure sodium lamp. At the same time, the igniter 16 provides at itsoutputs 26 periodic pulses 42 having an amplitude of 2500 to 4000 voltsand a duration of at least one microsecond. The periodic pulses 42 aresubstantially synchronized with the peaks of the AC voltage 40. Leadcircuit ballasts require one pulse per half cycle of the AC voltage, asshown in FIG. 4, while lag circuit ballasts require one pulse per cycleof the AC voltage. After the discharge is established and the lamp 30 isfully warmed up, the current drawn by the lamp 30 reduces the AC outputvoltage 40 of the lamp ballast 18 and periodic pulses 42 are no longerprovided.

As noted above, the high voltage pulse 38 is generated when the startercontacts are opened and, thus, occurs at a random time during the ACcycle of the lamp voltage 40 (not necessarily at the beginning of thecycle as illustrated in FIG. 4). Further, the illustrated pulse 38 isidealized without showing the normally occurring transient ringing.Typical lamp ballasts 18 include transformers and have highly inductiveoutput impedances. Prior to the time T_(o), a substantial current isdrawn from the lamp ballast 18 through the short circuit provided bystarter 10 via terminal 2, bimetal 7, rod 9 and terminal 3. When thestarter contacts are opened, the current drawn from the lamp ballast 18rapidly decreases and the inductance of the lamp ballast 18 generatesthe high voltage pulse 38. The energy provided by the high voltage pulse38, in combination with the periodic pulses 42 and the conductor 12, issufficient to form a discharge in the discharge lamp 30. For optimumlamp operation, the high voltage pulse 38 has an amplitude approximatelyequal to the amplitude of the periodic pulses 42 and a duration muchgreater than the duration of the periodic pulses 42. The high voltagepulse 38 is typically about 100 microseconds in duration.

The effect of the conductor 12 around the discharge tube 20 is toprovide electric field distortion such that the electric field near theelectrodes 22 is intensified within the discharge tube. The developmentof ionization in this region is thought to spread progressively alongthe inside surface of the arc tube 20 until a continuous path ofionization is produced between the two electrodes 22. When an ionizationpath is formed in which electron densities and temperatures aresufficiently elevated, the path is relatively highly conductive. Atxenon pressures below 300 torr, the ionization path absorbs additionalpower and increases in conductivity until an arc discharge is formed andthe lamp has been started. However, at xenon pressures in excess of 300torr, the initial ionization path does not absorb additional power andarc formation does not occur in the absence of the starter device 10, ora switching device of the type described in the aforementioned copendingapplication Ser. No. 139,310.

The voltage levels in the ballast system should not exceed the ratedvalues, typically about 2500 volts for standard high pressure sodiumlamp ballasts. The amplitude of the high voltage pulse 38 is given by Ldi/dt where L is the output inductance of the lamp ballast, di is thechange in current when the starter contacts are opened, and dt is thetime required for di to occur. Thus, the amplitude of the pulse can becontrolled either by controlling the current through the closed startercontacts or by controlling the speed at which the starter contacts open.In accordance with the present invention, it has been found that theamplitude of the pulse 38 can be further controlled via glow starterdevice 10 by selection of the glow-bottle gas and pressure; specificexamples were given hereinbefore.

It has been found that the product of the duration of the pulse and theamplitude of the pulse must be higher than a minimum value to obtainreliable lamp starting. Since it is desirable to limit the pulse 38 toapproximately 2500 volts, we have found that the duration of pulse 38must be long in relation to the duration of periodic pulse 42. It is tobe understood that, while the configuration shown in FIG. 3 is mostuseful to start and operate high intensity lamps containing noble gasesat pressures in excess of 300 torr, it can also be used to start andoperate lamps containing noble gases at lower pressures.

In operation, the lamp ballast provides AC power on output lines 24 andthe igniter provides periodic pulses of high amplitude and shortduration on output lines 26, as described hereinabove. Also, theconductor 12 promotes the formation of an ionization path within the arctube 20 as described hereinabove. In a cold, or quiescent condition, thebimetals 6 and 7 via segment contact 8 make a normally closed contactwith the rigid member, or rod, 9. Therefore, when power is applied atthe ballast circuit outputs, current flows through starter deviceterminal 2, bimetal 7, rod 9 and terminal 3. Upon the ballast shortcircuit being drawn through the bimetal 7, the I² R thereof issufficient to flex bimetal 7 for separating both of the connectedtogether bimetals 6 and 7 from the rod 9 to provide an open circuitthereat, whereby the current drawn from the ballast is rapidlydecreased. The rapid decrease in current drawn from the ballast causesthe highly inductive output of the ballast to generate a high voltagepulse which provides sufficient energy to initiate discharge in the lamp30 as hereinabove described and shown in FIG. 4. The current drawn bythe discharge lamp 30 flows through both of the connected togetherbimetals 6 and 7 by virtue of the illustrated circuit connection, andthe resulting I² R of the bimetals is sufficient to maintain the bimetalcontact segment 8 separated from rod 9, whereby the starter contactsremain in the open position. If for some reason the discharge lamp 30does not start when the device 10 contacts are open, no current is drawnthrough the bimetals, whereupon the strips 6 and 7 cool and reclose thecontact between segment 8 and rod 9. Heating of bimetal 7 again occurscausing the starter device contacts to open and provide another highvoltage starting pulse. Thus, the starting process is repeated until adischarge is initiated in lamp 30.

Accordingly, it is clear that the glow starter 10 of the presentinvention is a current device as opposed to the conventional voltagetype glow starters. Operation of the starter 10 is not a function of theopen circuit voltage, rather the I² R deflecting function is responsiveto short circuit current. The device works in circuits having low opencircuit voltages where more common glow bottle starter techniques havenot been able to be utilized.

Although the invention has been described with respect to a specificembodiment, it will be appreciated that modifications and changes may bemade by those skilled in the art without departing from the true spiritand scope of the invention. For example, the two bimetals may be formedfrom a single strip which is separated longitudinally for a substantialportion of its length; accordingly, the connection at one end would thenbe the unseparated portion of the strip.

I claim:
 1. A glow starter device comprising an hermetically sealedglass envelope having three lead-in wires extending therethrough; a gaswithin said envelope at subatmospheric pressure; a first bimetal withinsaid envelope mounted on a first one of said lead-in wires; a secondbimetal within said envelope mounted on an second one of said lead-inwires, said bimetals being electrically and mechanically connectedtogether at one end; and a rigid conductive member within said envelopemounted on a third one of said lead-in wires; said bimetals beingdisposed to make electrical contact with said rigid conductive member atthe connected-together end of the bimetals at normal room temperatureand in the absence of energization of the lead-in wires, and saidbimetals being operative to separate from said rigid conductive memberat a predetermined elevated temperature such as caused by electricalcurrent flowing through one or both of said bimetals.
 2. The starterdevice of claim 1 wherein the gas fill of said envelope is argon at apressure of about 2 torr.
 3. The starter device of claim 1 wherein saidrigid conductive member is a metal rod.
 4. The starter device of claim 3wherein said metal rod is tungsten.
 5. The starter device of claim 4wherein each bimetal is in the form of a flexible strip, and said stripsare connected together at one end by a segment of tungsten welded acrossthe strips, said tungsten segment being a contact member which isresiliently urged against said tungsten rod by said bimetals in thequiescent state of said device.
 6. A discharge lamp starting circuitcomprising, in combination:a ballast circuit having an input connectedto a source of AC line voltage and a power output; a glow starter devicehaving a first terminal connected to a first bimetal, a second terminalconnected to a second bimetal, and a third terminal connected to a rigidconductive member, said bimetals being electrically connected togetherat one end which makes a normally closed contact with said rigid memberin the quiescent state of said device; means connecting the second andthird terminals of said starter device across the output of said ballastcircuit; and means for connecting the first and third terminals of saidstarter device across the terminals of a discharge lamp; whereby uponinitial energization of said ballast, short circuit current through thesecond and third terminals of said starter device is operative to flexsaid second bimetal for separating said bimetals from said rigid memberto provide an open circuit thereat and a switching transient across thelamp, and upon starting of said lamp, the lamp current flow through thefirst and second terminals of said starter device is operative tomaintain said bimetals separated from said rigid member.
 7. The circuitof claim 6 wherein said lamp is a high pressure discharge lamp includingan hermetically sealed arc tube having electrodes sealed therein andconnected to respective terminals of the lamp, said lamp containing anoble gas or mixtures thereof having a pressure of greater than 100torr.
 8. The circuit of claim 6 wherein said ballast circuit provides anAC output and includes an igniter for providing periodic high voltagepulses substantially synchronized with the peaks of the AC output of theballast circuit, and each separation of said bimetals from said rigidmember in said starter device is operative to produce a high voltagepulse across said lamp which has a substantially longer duration thanthe periodic pulses of said igniter.
 9. The circuit of claim 8 whereinsaid starter device further includes an hermetically sealed envelopewithin which said bimetals and said rigid member are disposed, and saidenvelope is filled with a gas at subatmospheric pressure, the amplitudeof said pulse produced by said starter being controlled by the selectionof said gas and the pressure thereof to be about the same as theamplitude of the periodic pulses.
 10. The circuit of claim 7 whereinsaid lamp has a starting aid comprising a conductor coupled to one ofsaid electrodes and located in close proximity to the outer surface ofsaid arc tube.